Spheroidal alumina



Patented Jan. 19, 1954 SPHEROIDAL ALUMIINA James Hoekstra, EvergreenPark, Ill, assignor to Universal Oil Products Company, Chicago, 111., acorporation of Delaware No Drawing. Application November 21, 1951,Serial No. 257,634

This is contln atlon-in-part of my copending application Serial No.148,509, filed March 8, 1950, and issued as Patent No. 2,620,314 onDecember 2, 1952.

This invention relates to the manufactur of alumina particles and moreparticularly to a novel method of manufacturing alumina particles ofsubstantially spherical or spheroidal shape.

The use of alumina particles in substantially spherical or shape offersnumerous advantages, particularly when he alumina is used as anadsorbent, treating, refining or purifying agent, or as a catalyst orcomponent of a catalyst for: the conversion of organic compounds andstill more particularly for the conversion of hydrocarbons. When used asa fixed bed of packing materials in a reaction or contacting zone, thespheroidal shaped particles permit more uniform packing and therebyreduce variations in pressure drop through the bed and accordinglyreduce channeling which otherwise results in a portion of the bed hemcy-passed. Another advantage to the use of particles of this shape isthat the spheres contain no sharp edges to break or wear off duringprocessing or handling and, therefore, reduce the tendency to plug theprocess equipment. These advantages are magnified when the aluminaparticles are used as a moving bed, that is, when the particles aretransported from one zone to another by either the reactants or by anextraneous carrying medium. It is thus seen that the use of particles ofthis shape permits a "more efiective utilization of the alumina.

Another very important advantage to the manufacture of particles inaccordance with the present invention is in the matter of economics. Aprior method of obtaining alumina particles of uniform and shape and or"the desired hardness has been by means of a pilling operation. From thedescription to be hereinaf er set forth, it will be readily apparentthat the process of the present invention affords a considerably moreeconomical. method of manufacturing the alumina particles ofsubstantially uniform size and shape.

Extensive investigations have been conducted on the manufacture orsubstantially spherical or spheroidal alumina particles by the method ofpassing droplets of alumina-containing solutions, sols or other 'xturesinto an immiscible liquid. These investigations have shown thatsatisfactory alumina particles are not as readily manufactured by thimethod as are some other in- 6 Claims. (01. 252-448) organic oxideparticles as, for example, silica spheres. In order to obtainsatisfactory spherical particles of alumina, it is necessary to employ asol which will set to a gel within a desired time interval. For example,when adding a conventional precipitating reagent, such as ammoniumhydroxide, to an aluminum salt, such as aluminum chloride, aluminumnitrate, etc, a gelatinous precipitate sets immediately and, therefore,cannot be formed into the desired spherical shape by this method ofoperation. It has now been found that satisfactory alumina particles maybe manufactured by this method provided certain critical features areadhered to. [as wi l be set forth hereinafter, these features areextremeiy critical in order to obtain rigid particles which will notdissolve or crack during manufacture or use.

In one embodiment the present invention relates to a method ofmanufacturing alumina particles which comprises commingling an alaminasol with a Weak base having a strong buifering action at a pH of fromabout 4'. to about 10 and an increased rate of hydrolysis at anincreased temperature without the evolution of gas, passing theresultant mixture in finely divided form into a water immisciblesuspending medi um maintained at an elevated temperature and retainingthe alumina $01 in said suspending medium until gelation occurs.

From the above embodiment it will be noted that, in accordance with thepresent invention, an alumina sol is commingled with a weak base ofspecific properties. This is one of the essential features of thepresent invention because this permits the preparation of a mixture ofalumina sol and base which will not set immediately to a gelatinous massbut which, on the other hand, will set into a gel within a reasonabletime. This time diiiercntial permits passing the mixture into asuspending medium so that the sol may assume the desired shape duringpassage through the suspending medium. The weak base for use inaccordance with the present invention must be water soluble and have astrong buiiering action at a pH of from about 4 to about 19 andpreferably of from about 5 to about 8.5 and this means that the base, ata pH within this range, may be commingled with a relatively large amountof acidic material and still more substantially decrease the pH of themixture. Therefore, for example, the conventional ammonium hydroxidecannot be used because it does not meet this requirement and, ashereinbefore set 3 forth, a gelatinous mass is immediately precipitated.

It will be noted that the weak base for use in accordance with thepresent invention is defined as having a strong buffering action at a pHof from about a to about 10. When measured at room temperature, thisstrong bufiering action is more pronounced at a pH.of from about 4 toabout 6. However, it is believed that at a higher temperature andparticularly at the temperature utilized in the forming zone of thepresent process, this buffering efiect may be more pronounced at ahigher pH range which may be from about 5 to aboutlO. For example, ithas been found that, when different samples of a weak base comprisinghexamethylene tetramine were heated in admixture with differentconcentrations of hydrochloric acid and then cooled, the pH of thesamples varied from the pH of similar samples which had not been soheated. It is appreciated that pH determinations are now made at roomtemperature because of the ease in so doing and the diflicultiesencountered when attempting tomeasure the pH at elevated temperatures.Therefore, it is understood that the pH range used in the presentspecification. and claims is intended to include determinations made atroom temperature as now generally practiced or at an elevatedtemperature by a modified analytical method.

The weak base also must have an increased rate of hydrolysis at anincreased temperature without the evolution of gas. This permitscommingling the weak base with the alumina. sol at normal temperatures,which generally will be below about 110 F., without precipitation of agelatinous mass. Upon heating the mixture to an elevated temperature offrom about 120 to about 220 F., the alumina sets to a gel and thispermits forming spheroidal alumina in the manner to be hereinafter setforth in detail. For example, it has been been found that uponcommingling the alumina sol with a weak base comprising hexamethylenetetramine, gelation occurred in about 3-5 hours at room'temperar ture ofabout 70 F. However, when heated to a temperature of 190 F., gelationoccurred in 1 to 2 seconds. Still further, when refrigerated to atemperature of from about 32-35 F. gelation did not occur in 5 days.Therefore, it is important that the weak base has the property of anincreased rate of hydrolysis at an elevated temperature so that thealumina sol and base may be commingled at a normal temperature withoutgelation but that gelation will occur within a reasonable time whenpassed into a heated suspending medium in the manner to be hereinafterset forth.

Still further, another requirement of the weak base is that it will notresult in the evolution of gas at the elevated temperature employed inthe process. For this reason, ammonium carbonate cannot be employedbecause it will result in the evolution of carbon dioxide which willdisrupt the formation of the alumina spheres.

As another feature of the present invention, the suspending medium iswater immiscible and is maintained at an elevated temperature in orderto obtain the desired gelation within a reasonable time. As stillanother feature of the present invention, the particles are aged in abasic medium before being contacted with water. This is contrary to theresults obtained when preparing some other inorganic oxide particles as,for example, in the. preparation of silica spheres. One method used inthe preparation of silica spheres is to utilize a body of Water beneaththe suspending medium to thereby transport the silica spheres from theforming zone to subsequent treatment. When this method is employed inthe preparation of alumina spheres, the spheres dissolve in the waterand thus are destroyed.

From the above brief description, it is apparent that the manufacture ofsatisfactory alumina particles by this method differs from themanufacture of some other inorganic oxide particles and, therefore,requires certain critical features as will be set forth hereinafter indetail.

In one of the preferred methods of preparing the alumina sol, aluminummetal is added to an aqueous solution of aluminum chloride and thismixture is subjected to heating and digesting at its boiling point. Ingeneral, this tem perature will range from about to about 20 F.- Thetime of heating and digesting will depend upon the purity and particlesize of the aluminum metal employed. With a substantially pure metal,the time may range from about 24 to about 72 hours or more and this timeof heating and digesting will be reduced as impurities in the metal areincreased. However, when the alumina is used for special preparationswhich will not permit the presence of impurities, it is preferred toutilize substantially pure aluminum metal, as well as substantially purealuminum chloride. In some cases, the time of heating and digesting canbe expedited by adding an extraneous metal which is lower than aluminumin the electromotive series of elements but here again the extraneousmetal must be one that will not introduce an undesired impurity into thefinal alumina. Another method of expediting the heating and digesting isto pass oxygen into the mixture, as this serves to oxidize the hydrogenand thereby expedite the reaction.

The product resulting from the heating and digesting of the aluminumchloride and aluminum metal is referred to in the present specificationsand claims as an alumina sol. The exact chemical composition of thisproduct has not been definitely established but it may be representedapproximately by the formula 4.5Al (OH) 3.A1C13 However, it isunderstood that the present invention is not limited to this specificcomposition, and that this product also may be referred to as asolution, colloidal solution, etc. In any event, this product containsless combined chlorine than is present in aluminum chloride (AlCls) andalso is readily soluble in water.

It has been found that, in the preparation of the alumina sol, the molratio of aluminum chloride to aluminum metal is preferably Within therange of from about 1:3 to about 1:5. Ratios of aluminum chloride toaluminum metal substan-.

tially outside of this range result in a sol which sets immediately intoa precipitate upon the addition of the weak base or in the formation ofgel spheres which are too soft and, therefore, unsatisfactory. Aluminumnitrate may be utilized in place of aluminum chloride but notnecessarily with equivalent results.

As hereinbefore set forth, for certain uses it is desired that thealumina be of extremely high purity. Therefore, it is within the scopeof the present invention to utilize other methods of preparing thealumina sol when the available aluminum metal contains an undesirableimpurity which will remain in the alumina spheres. In one method ofpreparing a suitable alumina sol, a solution of an aluminum salt andparticularly aluminum chloride may be electrolyzed in an electrolyticcell having a porous partition between the anode and the cathode. Anacid anion deficient aluminum salt solution may then be recovered fromthe cathode compartment. In still another method, a solution or analuminum salt may be subjected to treatment with an anion exchange agentwhich will remove some of the acid from the salt solution. It isunderstood, however, that these methods are not necessarily equivalentto that obtained by reacting the aluminum salt with aluminum metal.

The alumina sol as prepared in the above manher is a colorless toslightly yellow liquid and, as hereinbefore set forth, is readilysoluble in water. An aqueous solution of the sol is prepared whichcontains from about to about 35% by weight of alumina calculated asA1203. It has been found that certain sols having a concentration ofalumina above 35% set to a gel immediately upon commingling with thebasic reagent or may result in spheres which are brittle and crackeasily. On the other hand, certain sols having a concentration below 15%result in spheres which are too soft and, therefore, unsatisfactory.However, as will be hereinafter set forth, the amount of water in thesol may be varied depending upon the amount of water in the weak basewhich is commingled with the sol.

As hereinbefore set forth, an essential feature of the present inventionis in the selection of the weak base for commingling with the aluminasol in order to prepare a mixture which will set to a gel within areasonable time and which will form alumina spheres of desired rigidity.It has been found that a particularly suitable weak base for use inaccordance with the present invention is hexamethylen tetramine which isvariously referred to as methenamine, formin, etc., and is representedby the formula (CH2)GN4. Hexamethylene tetramine is readily prepared bythe reaction of formaldehyde with ammonia. It has been found thatapproximately a 30% solution is particularly satisfactory for ease ofhandling and, when the water in the alumina sol is within the rangeshereinbefore set forth, results in a final mixture of desired Watercontent. However, the hexamethylene tetramine solution may vary fromabout a 15% solution to about a 40% solution, the latter beingsubstantially the saturation point. However, as the amount of water isincreased in the hexamethylene tetramine solution, the amount of waterin the alumina sol must be decreased accordingly, and vice versa.

Weak bases are also formed by the reaction of ammonia with otheraldehydes such as acetaldehyde, proprionaldehyde, etc. but in view ofthe fact that these bases have a higher pH than the reaction product ofammonia with formaldehyde and that the pI-l obtained with ammonium andformaldehyde appears to be optimum, there does not appear to be anyadvantage in using aldehydes other than formaldehyde. However, in caseswhere the pH of the base is too high, acid may be commingled therewithin order to reduce the pH thereof to below about 8.5. Anothersatisfactory weak base comprises a mixture of ammonium acetate andammonium hydroxide, the mixture having a pH of below about 8.5. It isunderstood that these bases are not necessarily equivalent and also thatany other suitable weak base meeting the requirements hereinbefore setforth may be used in the present invention but not necessarily withequivalent results.

The solution of alumina s01 and the solution of hexamethylene tetramineare commingled and, in a preferred embodiment of the invention, dropletsare passed into an immiscible suspending medium. It has been found thatequal volumes of the sol solution and of the hexamethylene tetraminesolution are satisfactory but it is understood that this may varysomewhat. In general, however, the volume ratio of hexamethylenetetramine solution to alumina sol solution should be within the range offrom about 3:1 to about 111.5 when the sol contains 26% A1203 and thehexamethylene tetramine solution is 30% The use of a smaller amount ofhexamethylene tetramine solution tends to result in soft spheres While,on the other hand, the use of larger volumes of hexamethylene tetraminesolution results in spheres which tend to crack easily.

In the prior description, the preferred alumina sol has a chemicalcomposition which is repre sented approximately by the formula 4.5A1(OH) 3.A1C13 This alumina sol requires the use of a solutioncontaining from about 15% to about 35% by weight of alumina calculatedas A1203 which in turn requires the use of a volume ratio ofhexamethylene tetramine solution to alumina sol of from about 3:1 toabout 1:15.

In another embodiment of the invention the alumina sol may be preparedto contain a higher alumina to chlorine ratio by effecting the heatingand digesting of aluminum chloride in the presence of higherconcentrations of aluminum metal. These concentrations are within therange of from about 1:5 to about 1:? mol ratios of aluminum chloride toaluminum metal. In this embodiment it is important to use a more dilutesolution of the alumina sol, the solution containing from about 5% toabout 15% by weight of alumina calculated as A1203. With these lowconcentrations of alumina in the sol having a higher alumina to chlorineratio, satisfactory alumina spheres may be prepared by using smallervolume ratios of hexamethylene tetramine solution which may range fromabout 1:2 to about 1:20 volumes of hexamethylene tetramine solution tovolumes of alumina sol solution. Satisfactory spheres have been preparedwhen using 1 volume of hexamethylene tetramine solution to 10 volumes ofalumina sol prepared and diluted as hereinbefore set forth. However, itis understood that the volume concentrations of hexamethylene tetraminewhich may be used satisfactorily is dependent on the mol ratio ofaluminum chloride to aluminum metal used in the heating and digestingstep and on the concentration of alurnina in the diluted solution.Higher volumes of hexamethylene tetramine solution are required withhigher ratios of aluminum chloride to aluminum metal in the heating anddigesting step and with higher alumina concentrations in the aqueoussolution of the sol.

The two solutions are mixed preferably in a zone adapted to effectintimate mixing, and the mixture is then passed into the suspendingmedium. In a preferred method, the mixture is distributed in the form ofdroplets from a nozzle or orifice, the size of the nozzle determiningthe size of the alumina particles. When very small alumina particles aredesired, the mixture may be distributed from a rotating disk.

in order to allow sufficient time formixing and handling'of the twosolutions, thesolutions are preferably mixed and dropped atsubstantially room temperature. However, the suspending medium must beat an elevated temperature in order to obtain gelation within thedesired time. The temperature of the suspending medium may range fromabout 120 to about 220 F. and preferably is within therange of fromabout 19 to about 205 F. Temperatures below 120 F. require setting timesthat are too long and would necessitate a body of suspending mediumwhich is excessive for practical purposes. On the other hand,temperatures above about 220 F. result in vaporization of the Water andaccordingly cracking of the spheres. The time in which the spheresremain in the suspending medium should be suflicient to form rigidspheres which will not crack Or become distorted when removed from thesuspending medium. It is understood that the time andtemperature ofsuspension will be correlated to obtain the desired rigid spheres andthat these factors are inversely related; that is, as the temperature isincreased the time may be decreased.

Any suitable water immiscible suspending liquid which will not vaporizeatthese temperatures may be employed. A particularly. suitablesuspending liquid comprises organic liquids such as kerosene, Nujol, andsimilar. materials which will allow the droplets to settle at. a ratesuch that the alumina sets into a firm hydrogel during its passagethrough the fluid medium. While itv is. within the scope of the presentinvention to use a suspending liquid which is of higher density than thealumina spheres, in which case the alumina spheres rise upwardlythrough. the suspending liquid, this method generally is not aspreferred as is the use ofa suspending medium of lower density than thealumina spheres so that the spheres descend to the bottom of the formingzone.

In a preferred embodiment of the invention, the alumina spheres areremoved from the lower portion of the suspending liquid. Contrary tothe. experiences found with the other inorganic oxide spheres andparticularly silica spheres, the alumina spheres must not be contactedvwith water at this stage of operation. The alumina spheres are watersoluble and, therefore, would be destroyed upon being contacted withwater. It, therefore, is another important feature of the presentinvention that the alumina spheres must be aged rior to being contactedwith water. Furthermore, this aging must be in the presence of a basicmedium and this, as hereinbefore set forth, comprises another essentialfeature of the present invention.

Another advantage to the process of the present invention is thatspheres of differentdensities may be obtained by varying the aging. Forexample, when spheres of a density greater than about 0.7 are desired,these spheres, being re,- ferred to herein as high density spheres, thealumina spheres are aged ina weak base of the same characteristics asthe weak base-originally used in preparing the spheres. This aging iseffected at a temperature of from about 150 to about 212 F., preferably.of from about 190 to about 210 F., for a period of at least hours andpreferably of from about 16 to about 24-hours or more. Thus, in apreferred embodiment the alumina spheres are aged in hexamethylenetetramine at a temperature. and for a time as hereinbefore set forth.

When spheres of intermediate or high densities are desired, the sphereshaving densities of above about 0.5, the alumina spheres may be aged forat least 10 hours at a temperature above about F. in an oil whichpreferably is the same as the suspending medium and. then in ammoniumhydroxide solution for at least 10 hours. The exact density of thespheres will depend upon the concentration and temperature of theammonium hydroxide solution, higher densities being obtained with lowerconcentrations and with lower temperatures. However, it is understoodthat the spheres must be aged first in the oil for at least 10 hoursbefore being aged in the ammonium hydroxide solution because otherwisethe spheres will become soft or will crack.

When low density spheres are desired, that is, spheres having a densityof below about 0.5, the alumina spheres are aged in an oil of the typeused as the suspending medium at an elevated temperature and then inammonium hydroxide solution at an elevated temperature. The elevatedtemperature is above about 125 F. and generally will not be greater thanabout 220 F. The time of aging is at least 10 hours in each case and theammonium hydroxide solution preferably contains about 4 to 5% ammonia.The density of the spheres is again determined by the concentration ofthe ammonium hydroxide and, therefore, higher density spheres areobtained when a less concentrated solution or lower volume of solutionis used. The variations in density due to variations in temperature andammonia concentrations are further illustrated in the examples appendedto the present specification.

In another embodiment of the invention the spheres may be aged at anelevated temperature in the presence of the suspending medium for aperiod of at least 10 and preferably of from about 16 to 24 hours ormore at an elevated temperature of from about to about 212 F. andpreferably of from about to about 210 F. In effect this may beconsidered as aging in a basic medium because the alumina spheres willcontain hexamethylene tetramine and, therefore, are being aged in thepresence of this reagent.

Fromthe above description it is apparent that the preparation ofsatisfactory alumina spheres requires the use of a weak base of specificrequirements and also aging of the spheresin a basic medium prior tocontacting them with water. These features are extremely critical inorder to prepare substantially spherical or spheroidal particles ofsatisfactory rigidity.

After the aging treatment, the spheres may be washed in any suitablemanner. A particularly satisfactory method is to wash the spheres bypercolation, either with upward or downward flow of water, andpreferably with water containing a small amount of ammonium hydroxideand/or ammonium nitrate. After washing, the spheres may be dried at atemperature of from about 200 to about 600 F. for 2 to 24 hours or moreor dried at this temperature and then calcined at a temperature of fromabout 800 to about 1400 F. for 2 to 12 hours or more, and then utilizedas such or composited with other catalytic components. It is preferredthat the spheres be dried slowly and also that the drying be effected ina humid atmosphere because this has been found to result in lessbreakage of the spheres. In another embodiment of the invention thespheres maybe treated with other catalytic components prior to thedrying or drying and calciningoperations, and the final catalystcomposite then may be subjected to further drying and calcining asdesired.

In another embodiment of the invention and particularly when preparinghigh density spheres, the spheres may be given a quick wash, dried at atemperature of from about 200 to about 300 F. in a high humidityatmosphere, calcined at a temperature of from about 800 to about 1400 F.and further washed, preferably with water containing ammonium hydroxideand/or ammonium nitrate. Care must be exercised in preventing thespheres from absorbing moisture from the air which will occur before thespheres are subjected to high temperature drying, and this isparticularly applicable to the high density spheres. Therefore, it ispreferred to dry and calcine the high density spheres immediately afteraging without permitting the spheres to cool.

As hereinbefore set forth, the alumina spheres may be used as anadsorbent or refining agent to treat organic compounds and also areparticularly satisfactory for use as a component in catalysts. Thesespheres are particularly suitable for use as a component in the recentlydiscovered platforming catalyst which comprises alumina, from about0.01% to about 1 by weight of platinum and from about 0.1% to about 8%by weight of combined halogen. Another particularly suitable catalystcomprises alumina composited with from about 5 to about by weight of acompound and particularly an oxide of one or more elements in the lefthand columns of groups 4, 5 and 6 of the periodic table, which catalystsare utilized in reforming, hydrogenation, dehydrogenation,dehydrocyclization, etc. of hydrocarbons or other organic compounds.Typical catalysts of this type include aluminachromia,alumina-molybdena, alumina-vanadia, etc.

The platforming catalyst is utilized for the treatment of a gasoline ata temperature of from about 800 to about 1100 F. at a superatmosphericpressure of from about 100 to about 1000 pounds per square inch in thepresence of hydrogen. Dehydrogenation reactions are efifected attemperatures of from about 800 to about 1200 F. and usually at moderatesuperatmospheric pressures which are below about pounds per square inch.Hydrogenation reactions are effected at lower temperatures and higherpressures which generally may range from about 200 to about 600 F. andpressures of from about 200 to about 1000 pounds or more per squareinch.

The alumina spheres may be used as contacting agents or as treating orrefining agents for organic compounds and, thus, may find utility ineffecting dehydration reactions, dehydrohalogenation reactions,desulfurization reactions, etc.

The fohowing examples are introduced to illustrate further the noveltyand utility of the present invention but not with the intention ofunduly limiting the same.

Ensem le I 10 Example I I From the results of Example I it appearednecessary to form an alumina sol containing a higher ratio of aluminumto chloride ion than is contained in aluminum chloride. Such a sol wasprepared by placing grams of aluminum metal cuttings in a solution of241 grams of aluminum chloride hexahydrate in 600 ml. of water, andheating the mixture. This resulted in an aqueous sol containing about 16equivalents of aluminum and 3 equivalents of chloride ion in a volume of800 ml.

Ammonium hydroxide was added to the alumina sol as prepared in the abovemanner and here again a gelatinous precipitate set immediately.

Example III The alumina sol as prepared according to Example II wasutilized in the preparation of satisfactory substantially sphericalparticles in the following manner. A solution of 30% by weight ofhexamethylene tetramine in water was mixed in about equal proportionswith another portion of the sol as prepared in Example II. This resultedin a slightly viscous mixture which slowly set to a firm gel. Attemptsto pass droplets of this mixture into an oil bath proved impracticalbecause the sol would not set into a firm gel during passage through theoil. However, upon heating the oil bath to a temperature within therange of from about to 210 EH, firm hydrogel spheres were formed.

Example IV The spheres formed according to Example III were placed inwater in an attempt to wash the spheres. However, the spheres dissolvedin the water and, therefore, it was apparent that the spheres neededsome treatment before being contacted with water.

An attempt was then made to dry the spheres, either partially orcompletely, prior to washing, but this method proved unsatisfactorybecause, even after partial drying, the spheres were still soluble inwater. Complete drying resulted in fracturing of the spheres.

Example V It was found that aging of the spheres formed according toExample III in a concentrated solution of hexamethylene tetramine (50 to100 grams of hexamethylene tetramine per 100 m1. of water) at atemperature of from about 195 to about 210 F. for 16 hours resulted inthe formation of white opaque spheres. The spheres aged in this mannerwere then washed with water and it was found that these spheres did notdissolve in the water but retained their rigidity.

Example VI A continuous process for the manufacture of sphericalparticles was developed. Several batches of alumina sol (26-28% A1203)were prepared in substantially the same manner set forth in Example II.A hexamethylene tetramine solution was prepared by adding sufiicientwater to 291 grams of hexamethylene tetramine to form 1 liter ofsolution. The sol and solution were passed, each at a rate of 12 cc. perminute, into a small mixer having a bafile rotated by means of a motor.Droplets were emitted from the bot tom of the mixer into a formingchamber of 2 inch diameter and 5 feet long. The tip of the nozzle fromwhich the mixture was dropped was about 2 mm. in diameter, and-theforming chamber was filled with Nujol up to about 1 inch below thenozzle tip. The forming chamber was maintained at a temperature of 195Fjby-means of electrical heating elements surrounding the chamber. Thedroplets assumed substantially spherical shape during passage throughthe Nuiol and were removed from the lower portion of the forming chamberby means of a stream of Nuiol maintained at a temperature of 195 F. TheNujol stream containing the spheres was passed into another zone inwhich a level of Nujol was maintained. A basket was submerged beneaththe level-of Nujol in this second zone and this basket served to collectthe spheres and also to revent the spheres from contacting theatmosphere. The second zone likewise was maintained at a temperature ofabout 195 F. When approximately 1100 grams of alumina spheres,calculated on a dry basis, were accumulated, the basket was removed fromthe second zone and the s heres were aged in Nujol at substantially thesame tem erature for 16 hours.

The partially aged spheres were then further aged in an ammoniumhydroxide solution, formed by adding sufficient water to 1200 m1. ofconcentrated ammonium hydroxide to cover the s heres. for 24 hours at atemperature of 203 F. The aging solution was then drained and thespheres were washed with water containing a small amount of ammoniumhydroxide (20 ml. of ammonium h droxide per gallons of water). Thewashed spheres were then partially dried at a tem erature of 248 F. inan atmosphere of high humidity. The dried spheres were immediatelycalcined thereafter at a temperature of 1200 F. As hereinbefore setforth the spheres were not permitted to cool between the drying andcalcining steps, because spheres allowed to cool before calcining tendto absorb moisture which causes them to crack. The resultant spheres hada diameter of about 3 2' inch and a density of 0.49. The alumina spheresprepared in the above manner were well formed, rigid particles and couldbe exposed to the atmosphere and washed with water without substantialbreakage.

Example VII Satisfactory alumina spheres were prepared from an aluminasol. prepared in substantially the same manner as described in ExampleII, and utilizing an ammonium acetate-ammonium hydroxide mixture as theweak base. An ammonium acetate solution was prepared by dissolving 150grams of ammonium acetate in 100 ml. of water and regulating the pH to8.4 with ammonium hydroxide. This ammonium acetateammonium hydroxidesolution and the alumina sol solution were supplied to a continuousmixer. and droplets of the resultant mixture were passed into a body ofNuiol maintained at a temperature of about 203 F. The resultant sphereswere subseouently aged in a 5% ammonium hydroxide solution atapproximately 207 F. for 5 hours, after which the mixture was allowed tocool to room temperature. The spheres were withdrawn from the agingsolution and then were washed with water containing a small amount ofammonium hydroxide, after which the speres were dried in a humidatmosphere at 230 F. for about 5 hours, then at about 285 F. for about 1/2 hours and finally at'about 300 F. for 3 hours. The spheres weresubsequently-calcined at 1200 F. for 3 hours.

.The'spheres formed in the abovemanner were well formed and rigid sothat they could be handled further without undue breakage.

I claim as my invention:

1. A method of preparing substantially spherical alumina particles whichcomprises forming alumina sol, commingling said sol with water to form amixture containing from about 15% to about 35% by weight of alumina,separately commingling ammonium acetate with water and adding ammoniumhydroxide thereto in an amount to regulate the pi-I of the resultantmixture to below about 8.5, commingling said ammonium acetate-ammoniumhydroxide solution with the alumina sol at a temperature below about F.and maintaining the mixture at below about 110 F. to prevent gelationthereof, thereafter passing droplets of the ungelled mixture into an oilbath maintained at a temperatureof from about to about 220 F.,retainingthe droplets in said oil bath until the droplets set tohydrogel spheres, and immediately thereafter aging said hydrogel spheresin a basic medium.

2. The method of claim 1 further characterized in that said aging iseffected in the presence of an ammonium acetate-amnionium hydroxidesolution.

3. The method of claim 1 further characterized in that said aging iseffected first in oil at a temperature of from about 120 to about 220 F.for at'least 10 hours and then in ammonium hydroxide solution for atleast 10 hours.

4. In the preparation of shaped alumina particles wherein an alumina solis introduced into a water-immiscible suspending medium for gelationtherein, the method which comprises commingling with the alumina sol anammonium acetate-ammonium hydroxide solution having a pH value belowabout 8.5, maintaining said sol and solution at below gelationtemperature during the commingling thereof and until introductionthereof to said suspending medium, introducing the resultant ungelledmixture in the form of droplets into said suspending medium andmaintaining the latter at a gelation temperature of from about 120 F. toabout 220 F., and retaining the droplets in the suspending medium untilgelation occurs.

5. In the preparation of shaped alumina particles wherein an alumina solis introduced into a water-immiscible suspending medium for gelationtherein, the method which comprises commingling with the alumina sol anammonium acetate-ammonium hydroxide solution having a pH value belowabout 8.5, maintaining said sol and solution in unheated state duringthe commingling thereof and until introduction thereof to the suspendingmedium to prevent gelation of the mixture before entering the suspendingmedium, introducing the resultant ungelled mixture in the form ofdroplets into said suspending medium and maintaining the latter at agelation temperature of from about 120 F. to about 220 F., and retainingthe droplets in the suspending medium until gelation occurs.

6. Inthe preparation of shaped alumina particles wherein an alumina solis introduced into a water-immiscible suspending medium for gelationtherein, the method which comprises commingling with the alumina sol anammonium acetate-ammonium hydroxide solution having a pH value belowabout 8.5, maintaining said sol and solution at a temperature belowabout 110 F. during the commingling thereof and until introductionthereof to the suspending medium to prevent Lgelation of the mixturebefore entering the suspending medium, introducing the resultantungeliec'i mixture in the form of droplets into said suspending mediumand maintaining the latter at a gelation temperature of from about 120F. to about 220 F. and retaining the droplets in the suspending mediumuntil gelation occurs.

JAMES HOEKSTRA.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Heard Oct. 6, 1942 Weiser Aug. 2'7, 1946 Marisic et a1. Dec.27, 1949 Pierce et a1 Feb. 5, 1952

1. A METHOD OF PREPARING SUBSTANTIALLY SPHERICAL ALUMINA PARTICLES WHICHCOMPRISES FORMING ALUMINA SOL, COMMINGLING SAID SOL WITH WATER TO FORM AMIXTURE CONTAINING FROM ABOUT 15% TO ABOUT 35% BY WEIGHT OF ALUMINA,SEPARATELY COMMINGLING AMMONIUM ACETATE WITH WATER AND ADDING AMMONIUMHYDROXIDE THERETO IN AN AMOUNT TO REGULATE THE PH OF THE RESULTANTMIXTURE TO BELOW ABOUT 8.5, COMMINGLING SAID AMMONIUM ACETATE-AMMONIUMHYDROXIDE SOLUTION WITH THE ALUMINA SOL AT A TEMPERATURE BELOW ABOUT110* F. AND MAINTAINING THE MIXTURE AT BELOW ABOUT 110* F. TO PREVENTGELATION THEREOF, THEREAFTER PASSING DROPLETS OF THE UNGELLED MIXTUREINTO AN OIL BATH MAINTAINED AT A TEMPERATURE OF FROM ABOUT 120* TO ABOUT220* F., RETAINING THE DROPLETS IN SAID OIL BATH UNTIL THE DROPLETS SETTO HYDROGEL SPHERES, AND IMMEDIATELY THREREAFTER AGING SAID HYDROGELSPHERES IN A BASIC MEDIUM.